Use new eleventyExcludeFromCollections feature in 0.8.0: https://www.11ty.io/docs/collections/#option%3A-exclude-content-from-collections

sitemap.xml.njk

@@ -1,16 +1,14 @@
 ---
 permalink: /sitemap.xml
-excludeFromSitemap: true
+eleventyExcludeFromCollections: true
 ---
 <?xml version="1.0" encoding="utf-8"?>
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
 {%- for page in collections.all %}
-{%- if not page.data.excludeFromSitemap %}
   {% set absoluteUrl %}{{ page.url | url | absoluteUrl(metadata.url) }}{% endset %}
   <url>
     <loc>{{ absoluteUrl }}</loc>
     <lastmod>{{ page.date | htmlDateString }}</lastmod>
   </url>
-{%- endif %}
 {%- endfor %}
 </urlset>